After completing his mission in South Korea, a North Korean spy (Park Sung Woong) hopes to return to his motherland to see his son Myung-Hoon (T.O.P) and daughter Hye-In (Kim You Jung) again, but he is betrayed by his own country and killed by a South Korean agent.

In North Korea, his children Myung-Hoon and Hye-In are sent off to a forced labor camp. Later, high ranking North Korean military official Sang-Chul (Jo Sung Ha) visits Myung-Hoon at the labor camp. He offers Myung-Hoon a deal. If Myung-Hoon successfully carries out a spy mission in South Korea, Myung-Hoon and his sister will be released from the labor camp. Myung-Hoon accepts the deal and undergoes 2 years of intense training.

Myung-Hoon finally arrives in South Korea under the guise of a North Korean defector. He is adopted by a South Korean couple who are actually North Korean spies and enrolls at a local high school. At his new high school, Myung-Hoon sits next to a girl named Hye-In (Han Ye Ri). She has the same name as his younger sister. Hye-In is the frequent target of bullies in their class, but Myung-Hoon falls for her. Meanwhile, Myung-Hoon finally receives his mission from North Korea. He is to locate a North Korean spy with the nickname of Big Dipper (Jung Ho Bin) and take him out. Meanwhile, a power struggle ensues in North Korea with the failing health of dictator Kim Jung-Il and Myung-Hoon quickly becomes a liability.